1. Field of the Invention
The present invention relates to a cross-linkable composition crosslinkable by Real Michael Addition (RMA) reaction comprising a latent base catalyst composition and to an improved catalyst composition for use in crosslinkable compositions, in particular RMA cross-linkable compositions, which catalyst composition provides improved pot life. The invention further relates to the use of said catalyst composition for curing crosslinkable compositions, in particular cross-linkable coating compositions, at low temperatures, to a process for preparing a coating composition and for preparing a coating and to coatings obtainable by said process.
A composition crosslinkable by Real Michael Addition (RMA) reaction comprises a component with at least 2 activated unsaturated groups and a component with at least 2 acidic protons C—H in activated methylene or methine groups, that can react and crosslink to each other in the presence of a strong base catalyst. The advantage of RMA crosslinkable compositions in general is that they have a very high curing reaction speed even at room temperature, and can give good film properties, while requiring low VOC levels.
The crosslinkable composition is a 2-K system, which implies that the crosslinking catalyst is added to and mixed with the crosslinkable components shortly before use. From the moment of mixing, the crosslinking composition is potentially reactive and may start to crosslink. Such crosslinking compositions can be used only relatively shortly before the extent of viscosity build-up is such that the composition can no longer be used well. This effective use time is called the potlife. The potlife is generally considered ended when the viscosity becomes unacceptably high. It depends on the criticality of the application what viscosity is unacceptable. For example spraying is more critical than brushing. A central challenge in developing coating systems, especially those that are cured at low to moderate temperatures, is achieving a good balance between on one hand rapid hardening and crosslinking during application (also referred to as “curing” or “drying”) and on the other hand maintaining long pot lives, i.e. the ability to work with the paint without problems for a reasonable amount of time after preparation, at least an hour, but preferably much more.
For high solids systems containing less solvent to evaporate upon application, this challenge is significantly greater than for low or medium solids systems containing more volatile solvent. High solids systems are preferred or required by law to reduce environmental burden, and/or exposure of the painter to harmful solvent vapours. The preferred solids content is at least 55 wt % (dry weight after crosslinking relative to the total weight of the crosslinking composition), preferably higher.
Coatings for applications in decorative, vehicle refinish, metal, plastic, marine or protective coatings e.g. require several hours of potlife enabling the applicator to bring the paint composition on a substrate in a well-controlled manner. The viscosity and low solvent content requirements for high solids systems force the resin designer to select resins of lower molecular weight and/or lower glass transition temperatures that will require more reaction with a crosslinker to raise the Tg of the network to levels corresponding to a dry film, in the case of high solids paints. The lower amount of solvent used will create less “physical drying” effects of the film (physical hardening/reduced plastization due to the loss of solvent) than in paints using more volatile solvents, and also, the increase of the reaction rate going from paint to applied coating is less, because the increase of concentration of the reactive groups through the loss of solvent is less helpful. All these phenomena add to the problem that for high solids systems, a combination of fast drying and long pot life is very difficult to achieve, and much more so than in the case of medium or low solids systems.
A latent base crosslinking catalyst is used to increase this potlife, while allowing fast drying. A latent base catalyst becomes active predominantly only when the composition is applied, for example as a coating.
2. Description of the Related Art
The above described problem has been addressed by Noomen in Progress in Organic Coatings 32 (1997) 137-14 describing the use of latent base catalysed Michael addition as crosslinking reaction for high-solids polymer coating compositions of low VOC. Noomen describes several examples of crosslinking catalysts with the required basicity, for example the amidine types (such as tetra-methyl-guanidine) 1,5-diazabicyclo (4,3,0) non-5-ene (DBN), 1,8-diazabicyclo (5,4,0) undec-7-ene (DBN), tetra-butylammonium fluoride or in situ formed catalyst from a tertiary amine (like 1,4-diazabicyclo[2.2.2]octane: DABCO) with epoxy. Although such prior art catalysts might show quite acceptable curing behaviour in the RMA films, the short potlifes are too limited to get acceptable application times for rolling, brushing and spraying of the coatings, or the drying rate at lower curing temperatures is too low.
Noomen further describes that, although the film properties (such as durability when using malonate polyesters) looked promising, there were still severe shortcomings with this coating composition, in particular in the field of high solids coatings, because the curing under ambient and forced drying conditions revealed inhibition, speculatively assigned to the interaction of the carbon dioxide from the air with the strong base resulting in a tacky coating surface or inhibition by the interaction with the acidic groups of the substrate resulting in a low degree of cure or a minor adhesion. This was overcome by increasing the amount of catalyst but this resulted in a too short, unacceptable potlife, especially when using high solid formulations and in low temperature applications such as clear coats car refinishing, pigmented topcoats for marine, protective and aircraft, wood coatings, etc. Another problem was often the yellowing of the coating induced especially under stoving conditions.
EP0448154 (also from Noomen) describes to use certain carboxylic acids as a blocking agent for a strong basic catalyst. Although a longer potlife can be achieved, the basic catalyst with carboxylic acids as described in EP448514 provides an insufficient dust- and touch-drying behaviour and a low through-drying especially at ambient conditions. The prior art catalyst does not provide workable potlifes when inhibition problems are to be avoided and does not provide fast free-to-handle coatings, in particular for high solid coatings. Furthermore, deblocking of the catalyst blocked with e.g. carboxylic acids was only applicable at high temperatures.